1. Technical Field
The present disclosure relates to a method for strip testing of MEMS (Micro Electro Mechanical System) devices, to a testing strip of MEMS devices, and to a MEMS device configured to be used with the method.
2. Description of the Related Art
MEMS devices are playing an increasingly important role in the electronic industry, especially in the consumer electronic field of portable electronics, thanks to the reduced size and power consumption.
As is known, a MEMS device includes one or more dice of semiconductor material (e.g., in the case of a MEMS sensor device, a first die integrating a mechanical sensing structure and a second die integrating a related electronic interface made as an ASIC—Application Specific Integrated Circuit), encapsulated in a package, which protects and covers the dice and provides suitable electrical connections to the outside, e.g., for soldering to an external printed circuit board.
Common packages are the so called BGA (Ball Grid Arrays) or LGA (Land Grid Array) packages, which offer reduced area occupation and high density of the electrical connections.
FIG. 1 schematically shows a MEMS device, denoted as a whole with reference 1, provided with a BGA or LGA package 2. Package 2 includes a substrate 3, having an internal surface 3a to which the dice of the MEMS device are attached, and an external surface 3b, which carries suitable electrical connection elements 4 to the outside of the package 2, in the form of an array of “balls” or “bumps” (in the case of BGA packages) or “lands” (in the case of LGA packages, as is the case shown in FIG. 1). The substrate 3 is usually made of a multi-layer structure, composed of several layers of conductive material (generally metal) separated via dielectric layers; electrical traces are provided through the substrate 3 to connect the dice to the external electrical connection elements 4. A covering and protection material, generally a mold compound 5, is provided on the substrate 3 and covers the dice, protecting them from the external environment.
In particular, in the example shown in FIG. 1, MEMS device 1 comprises a sensor die 6, including a micromechanical detection structure, and an ASIC die 7, including a related interface electronics. Dice 6 and 7 are stacked, with suitable electrical connections in the form of wires (using the so called “wire bonding” technique) designed to electrically connect sensor die 6 to ASIC die 7, and ASIC die 7 to the substrate 3; moreover, vias 8 and suitable traces are provided through the various layers of the substrate 3 to route the signals between the ASIC die 7 and the electrical connection elements 4 (these being either detection signals or power supply signals, or any other kind of signals exchanged between the MEMS device 1 and external devices). Clearly, other arrangements are possible for dice 6 and 7, which may be placed side-by-side on the substrate 3; or sensor die 6 may be attached to the ASIC die 7 with the flip-chip technique, with direct electrical connections being provided between the two dice.
In the semiconductor industry, testing of MEMS devices, in order to assess the electrical and mechanical performances of the finished products, accounts for an important part of the manufacturing costs, especially due to the amount of time and the expensive systems and apparatuses, which have to be provided for performing the testing operations. Testing generally envisages providing a stimulus (e.g., in the form of a physical stress) to a MEMS device and detecting an output electrical signal generated by the MEMS device in response thereto.
In order to reduce testing costs and increase the overall efficiency, so called “strip testing” procedures have been proposed, envisaging simultaneous parallel testing of a number of MEMS devices arranged in strips, according to matrix layouts, instead of separately testing single MEMS devices. These testing procedures allow to achieve a great productivity improvement and a reduction of the time used for testing, and thus a reduction of the final manufacturing costs.
In this connection, FIGS. 2a and 2b schematically show a strip 10 of MEMS devices, again denoted with reference 1 (each one being for example a sensor device as shown in FIG. 1, or any other kind of known MEMS device). MEMS devices 1 are in a matrix arrangement, aligned along a first and second directions x, y of a plane xy: strip 10 in the example has a main extension along the first direction x. Testing systems may be designed for parallel testing of several devices, for example of a group thereof, as shown enclosed by the dashed box in FIG. 2a. 
As depicted in FIG. 2b, the various MEMS devices 1 are enclosed in the same mold compound 5 and attached to the same substrate 3, after manufacturing (but before the final singulation step); therefore, it is considered that the substrate 3 includes a plurality of portions, each corresponding to a single MEMS device, each portion being separated by the others by a boundary region where the final cut during singulation will be performed.
However, especially in the case of MEMS sensors, parallel testing of devices arranged in strips implies some difficulties, due to the need to carry out suitable physical stimulation of the various sensors during testing (e.g., providing a test acceleration for acceleration sensors, or a test pressure for pressure sensors), and particularly due to the fact that the stresses acting on the devices in strip form are different from the stresses acting on the single devices, separated from the others. Moreover, the various devices in the strip are to be electrically insulated, in order to perform electrical tests on the individual devices.
Physical stimulation of the various devices during testing is achieved through the use of suitable testing equipments, configured to exert specific stresses on the devices, for example envisaging the use of support tables providing accelerations along a plurality of axes.
Various solutions have already been proposed in order to solve the problem related to the stresses acting on the devices while in strip form, and their electrical insulation.
In particular, a proposed solution envisages first the singulation of the various MEMS devices 1, and then their placing in a suitable carrier structure (or tray), provided with a plurality of housings, each adapted to house a respective singulated device. The housings in the carrier structure are arranged so as to define a strip of MEMS devices 1, which may undergo a parallel testing procedure.
This method is advantageous since no undesired stresses act on the singulated MEMS devices 1 during parallel testing operations. However, huge investments for preparing the support structures are needed to house the various MEMS devices 1, which have to be designed and manufactured for each possible package size; also, continuous maintenance of the carrier structures is conducted to assure correct alignment of the MEMS devices 1 during testing.
A further testing procedure has been already employed by the present Applicant, the so called “pre-cut” method, envisaging, as shown schematically in FIG. 3, sawing of the common substrate 3 from the external surface 3b and of part of the overlying mold compound 5, so as to create trenches (or openings, or cut-out portions) 11 extending through the whole substrate 3 and through part of the mold compound 5. In a way that is not shown in FIG. 3, these trenches 11 extend in the strip 10 both along the first and the second directions x and y, so as to define a continuous hollow portion, separating and surrounding MEMS devices 1.
This cutting process also electrically insulates the various MEMS devices 1 from each other and allows testing in strip-form of the same devices, which are still hold together via the residual portions of the mold compound 5; in particular, thickness of this residual portion (starting from the surface thereof not originally contacting the internal surface 3a of the substrate 3) is configured to have sufficient rigidity to achieve the result of holding together the MEMS devices 1 during handling and testing operations. Moreover, the resulting physical separation achieved between MEMS devices 1 in the strip 10 limits the amount of reciprocal stresses during testing.
The Applicant has realized that, although advantageous, this testing procedure suffers from some drawbacks.
In particular, in the “pre-cut” process the final full separation of the various MEMS devices 1 (the so called “singulation” operation) is carried out after their testing in strip form. The singulation process releases the stresses exerted by the residual portion of the mold compound 5 on the devices, thus offsetting the devices from the previously calibrated values; indeed, the pre-cut process leaves a certain amount of residual stress acting on the devices during the calibration step, and after singulation the offset distribution is widely spread, possibly driving a part of the population out of a specification.
This offset thus implies the need of providing a second testing procedure, after the singulation operation, designed to guarantee that all finished MEMS devices 1 are within the specified tolerance values.
The Applicant has realized that this second testing step represents a non-value adding process, since it is performed only because of the remaining stresses acting on the MEMS devices 1 during the strip testing/calibration procedures.
Especially when MEMS production volume is increased, the investments in strip testing equipments and second test equipments have also to increase correspondingly and may come to represent an important part of the overall manufacturing costs. Therefore, the need is clearly felt for a testing procedure that would allow testing of MEMS devices in strip form, while reducing or possibly avoiding the need of performing a further testing step after singulation, thus drastically improving the overall testing costs and times.